In an ordinary seat with a reclining back, the seat back typically pivots about an axis that is roughly at the intersection of the frames of the seat back and the seat base. When an occupant of the seat rests against the seat back as it reclines, the occupant's torso rotates about his or her hip joints. The separation between the pivot axis about which the seat back rotates and the user's hip joint pivot axis about which the user's back rotates results in the seat back sliding upwards relative to the user's back as the seat back is reclined backwards, and downwards as the seat back is raised towards the upright position. This is known as “shear.” For most occupants, this is not a major problem. The occupant simply moves his or her torso so as to allow it to shift relative to the seat back.
However, for persons of limited mobility, including many users of power wheelchairs, the necessary shift in position is less easy. There is therefore a need for a reclining seat back, especially for wheelchairs, in which the shear between the seat back and the user's back is reduced. Various solutions have been proposed, including mechanisms to bring the pivot axis of the seat back nearer to the user's hip joint pivot axis, as well as mechanisms for sliding the seat back cushion up and down relative to the seat back frame as the seat back reclines. It is believed that a novel, simple, and robust mechanism capable of translating a back support relative to a back frame would be desirable.